Variable demand air rectification plant with recycle



Sept. 20, 1966 M- LITVIN ET AL 3,273,349

VARIABLE DEMAND AIR RECTIFICATION PLANT WITH RECYCLE Filed Feb. 15, 19652 GAS PRODUCT GAS SUPPLY [UNIT SUPPLY BASE LOAD T0 CUSTOMQR d IAIRSEPARATION V A W PLANT UNIT V wvw-+ l LIQUEFIED NITROGEN STORAGE\CLEAN WASTE NITROGEN \SCAVENGING STREAM- WASTE NITROGEN TO ATMOSPHEREALLEN V. MUS/(A Emmy/kw 2Q OM7 ATTORNEY United States Patent 3,273,349VARIABLE DEMAND AIR RECTIFICATION PLANT WITH RECYCLE Milton Litvin, NewYork, N.Y., and Allen V. Muska,

Berkeley Heights, N.J., assignors to Air Reduction Company,Incorporated, New York, N.Y., a corporation of New York Filed Feb. 15,1965, Ser. No. 432,722

3 Claims. (CI. 6250) This invention relates to the economic design of anair rectification process including the utilization of the refrigerationcapacity of a stored liquid product of said process.

More specifically, this invention relates to an economic design of anair rectification process, saving a substantial part of the powerconsumption needed to supply the required refrigeration includingstorage of liquefied product of said process, in which the refrigerationcapacity is re covered from the liquefied product as a result of heatexchange with recycled nitrogen. The stored liquefied product meets peakdemands, which peak demands may exceed the steady supply capacity of theair rectification process.

Most often it is proposed to store a product, for example, oxygen, of anair separation plant in gaseous form. In the usual air separation plantthat supplies a customer by pipeline, the customers steady gas demand issupplied directly from the producing unit, While high customer demand issupplied by high pressure gaseous storage. Liquid storage has notgenerally been used in this service, in spite, of the lower capitalinvestment in comparison to gaseous storage. This is due to the factthat considerable power is needed to make liquid by refrigerating thegas. This refrigeration has been lost when the liquid is vaporized andput into the pipeline for supply to the customer. It is therefore anobject of this invention to make the cheaper liquid storage attractiveby utilizing the refrigeration capacity of the liquefied product in anefficient and effective manner within the air separation plant.

It is further an object of this invention to provide an economic airseparation process in which product gas flow surges demanded by thecustomer on the process are evened out.

It is further an object of this invention to effectively conserve therefrigeration eifect captured within a liquefied product of an airrectification process for use within the process so as to provide foreconomic use of power.

These and other objects and advantages of this invention will becomeapparent in connection with the more detailed disclosure set forthhereinbelow.

The stored liquefied product may be oxygen, as shown in the drawing tobe discussed hereinafter, or some other liquefied product of theprocess, such as nitrogen. Further description will be made in terms ofproduct oxygen, although this invention is not limited to such storedproduct.

In general, liquid product, for example oxygen, produced by an airseparation process, is pumped through a heat exchanger, where it isgasified. This pumping controls the operation of a variable capacitycompressor. Nitrogen, for example clean waste nitrogen, emanating froman air separation process is compressed by the variable capacitycompressor and condensed as a result of counter-current heat transferwith the gasifying oxygen. The nitrogen, just liquefied as a result ofheat exchange with the vaporizing oxygen, is stored and fed at a steadyrate to the air separation process to be used in any manner desired, forexample as reflux to the low pressure column. Effectively, the gaseousnitrogen stream recovers the refrigeration capacity of the gasifyingoxygen product before the product leaves the system. The dis- 3,273,349Patented Sept. 20, 1966 closed process can be utilized most effectivelywhere there is a large customer requirement for the product, be itnitrogen or oxygen or some other product, during periods of peakoperation, as will be apparent from the more detailed description inconnection with the drawing that follows.

The drawing shows schematically an air separation plant and an externalprocess and equipment therefor. The drawing shows liquid oxygen storage;however, as stated earlier, other liquefied products, such as liquidnitrogen, could be utilized in the same manner as is describedhereinafter.

The air separation plant unit 1 is steady state in its production ofliquid and gaseous products; that is, for substantial periods of time itproduces at a particular constant percent of capacity no matter what thecustomers demands are. Of course, from time .to time that percentage ofcapacity production may be altered. Said air separation plant unit 1supplies oxygen gas product, at 2, as a unit sup-ply base load to thecustomer. The steady state air separation plant unit 1 also producesliquid oxygen which is stored at 3 to fulfill the surge requirements ofthe customer. Of course, liquid nitrogen may be stored at 3, thisdescription of the process shown in the drawing being written in termsof oxygen storage only for purposes of illustration.

Clean waste nitrogen gas emanates from the air separation plant unit at4 in one pass of a multiple pass exchanger. The scavenging stream 5usually exhausts to the atmosphere. Although the description is madewith particular reference to the stream in pass 4 being clean wastenitrogen, other forms of nitrogen exhausted from the plant may beutilized. In the air separation plant 1 and external process of thedrawing, the product liquid oxygen stored in the storage vessel 3 isvaporized in a heat exchanger 7 to satisfy surge gas demands of acustomer, conserving the refrigeration value of the liquid oxygen byvaporizing said liquid oxygen against clean warm waste nitrogen gas 4running countercurrently.

The clean waste nitrogen 4 when it leaves the air separation plant is atatmospheric pressure and is warm. A typical operating temperature ofsaid clean waste nitrogen would be 86 F. The clean waste nitrogen gas 4,after being compressed in the variable capacity gas compressor 9 ispassed countercurrently to the vaporizing oxygen through pass 15 of heatexchanger apparatus 7. The clean warm Waste nitrogen gas transfers itsheat in the heat exchanger to the cold liquid oxygen in the adjacentpass 16, gasifying said liquid oxygen. The nitrogen gas is condensed asa result of the heat exchange with the cold liquid oxygen. The liquidoxygen is pumped to the heat exchanger 7 from the liquid storage vessel3 by a pump 6 in response to the variable surge demands of the gascustomer. The pump 6 may be linked through a motor drive interlockcontrol 8 to the variable capacity nitrogen compressor 9. (The nitrogengas in line 4 is compressed by the compressor 9 when product oxygen ispumped by pump 6, as a result of inter-lock control 8.) The liquefiedwaste nitrogen is discharged from the heat exchanger 7 and passesthrough a throttle valve control 17, at lower pressure, into a liquefiednitrogen storage vessel 11. In passing through the throttle valve 17,the liquid nitrogen undergoes a Joule-Thomson expansion in which someliquid is vaporized. This cold vapor, plus other cold vapor resultingfrom the effects on the liquid nitrogen of heat leak into the storagevessel 11, is vented from the storage vessel through valve 14, asdetermined by storage pressure control mechanism 18, and passedcountercurrently through pass 19 in heat exchange relationship inexchanger 7 to the warm waste nitrogen gas stream in pass 15. Thus, aportion of the heat in said waste nitrogen gas stream in pass 15 isremoved as a result of heat exchange with the cold gas in pass 19,adding to the saving of the refrigeration capacity originally taken fromthe gasifying oxygen. More efficient and complete liquefying of thewaste nitrogen gas and conserving of refrigeration and thus power iseffected. The cold vented gas from storage container 11 is thus warmedin exchanger 7 and then combined in gaseous form with the clean warmwaste nitrogen gas emanating in line 4 from the air separation plantunit 1. The liquefied waste nitrogen storage vessel 11 dischargesthrough a flow control valve 20 nitrogen liqued which is used in the airseparation plant, for example, as reflux to the low pressure orso-called oxygen column.

The variable capacity compressor is linked to the gasified oxygen supplyline 13 by a temperature capacity control 22. This control adjusts thespeed of the compressor 9, so that adequate waste nitrogen is suppliedto the exchanger 7 to vaporize all of the liquid oxygen in pass 16 ofthe exchanger. Pressure control 21 maintains the pass 15 of exchanger 7at the proper pressure needed for the transfer of heat between the threeexchanger fluids. This pressure controller maintains this pressure bycontrol of throttling valve 17, as above.

In a typical operation, the following process conditions will occur inthe heat exchanger.

Warm end T=86 I Mass flow ratios: Waste nitrogen to flash recycle toliquid 0xygen= 1.07 0.257 1.0 lb. waste nitrogen liquefied per lb.liquid oxygen=0.813.

The proposed process functions according to the product demanded by thecustomer. The refrigeration capacity of the liquid product is conservedby, in effect, transferring said refrigeration capacity to a wastenitrogen stream which is reintroduced back into the air separationplant. As a result of the liquefication of the clean waste nitrogen gasand then subsequent storage thereof, the refrigeration supply to the airseparation process is evened out.

The process described above is external to the air separation plant unit1 and is therefore independent of the particular cycle or process usedin the air separation plant, since all that is required is that the airseparation plant discharge some amount and form of nitrogen whichdischarge is standard. Therefore, the process described above can beused with all types of air separation processes or plants.

As stated previously, this invention is not limited to th storage ofliquefied oxygen product and a clean waste nitrogen stream emanatingfrom an air separation plant. As stated previously, oxygen product andclean waste nitrogen are used in this description only for purposes ofillustration of the invention. It should be understood that other storedliquefied products than oxygen (such as nitrogen) and other streamsemanating from an air separation plant than clean waste nitrogen (suchas a scavenging stream of nitrogen which is then purified) can beutilized according to the invention described hereinbefore. Furthermore,this invention is not limited to any method, process or apparatusdescribed hereinbefore, except as set forth in the following claims.

We claim:

1. A process for supply of gaseous oxygen from an air separation plantso as to meet varying demands comprising the steps of passing gaseousoxygen product at a substantially steady state from the plant to asource of demand, storing liquid oxygen product from said plant,vaporizing said liquid product in a vaporizer in response to demand,compressing waste nitrogen from said plant, cooling substantially all ofsaid compressed waste nitrogen by heat exchange with said liquid oxygenproduct being vaporized, throttling said compressed and cooled wastenitrogen to liquefy it, controllably applying said liquefied wastenitrogen to said plant to provide refrigeration therefor, and passingvapors from said liquified nitrogen in heat exchange with the compressednitrogen stream and then into mixture with the waste nitrogen beforesaid waste nitrogen is compressed.

2. A process for supply of gas product from an air separation plant soas to meet varying demands, comprising the steps of passing a gasproduct stream at a substantially steady state from the plant to asource of demand, storing liquid products from said plant, vaporizing itin a vaporizer in response to the demand, compressing a nitrogen gasstream from said plant, cooling substantially all of said last-mentionedcompressed gas by heat exchange with said liquid product being vaporizedthrottling said compressed and cooled nitrogen to liquefy it,controllably supplying said liquefied nitrogen stream to said plant toprovide refrigeration therefor, passing said vaporized product on to thesource of demand, and directing vapors emanating from said liquefiednitrogen in heat exchange with the nitrogen gas stream before saidstream is compressed.

3. A process as set forth in claim 2, the gaseous product stream andliquid products being oxygen.

References Cited by the Examiner UNITED STATES PATENTS 3,058,314 10/1962Gardner 6252 X 3,058,315 10/1962 Schuftan 62-52 3,195,316 7/1965 Maheret al. 6252 LLOYD 1,. KING, Primary Examiner.

1. A PROCESS FOR SUPPLY OF GASEOUS OXYGEN FROM AN AIR SEPARATION PLANTSO AS TO MEET VARYING DEMANDS COMPRISING THE STEPS OF PASSING GASEOUSOXYGEN PRODUCT AT A SUBSTANTIALLY STEADY STATE FROM THE PLANT TO ASOURCE OF DEMAND, STORING LIQUID OXYGEN PRODUCT FROM SAID PLANT,VAPORIZING SAID LIQUID PRODUCT IN A VAPORIZER IN RESPONSE TO DEMAND,COMPRESSING WASTE NITROGEN FROM SAID PLANT, COOLING SUBSTANTIALLY ALL OFSAID COMPRESSED WASTE NITROGEN BY HEAT EXCHANGE WITH SAID LIQUID OXYGENPRODUCT BEING VAPORIZED, THROTTLING SAID COMPRESSED AND COOLED WASTENITROGEN TO LIQUEFY IT, CONTROLLABLY APPLYING SAID LIQUEFIED WASTENITROGEN TO SAID PLANT TO PROVIDE REFRIGERATION THEREFOR, AND PASSINGVAPORS FROM SAID LIQUIDFIED NITROGEN IN HEAT EXCHANGE WITH THECOMPRESSED NITROGEN STREAM AND THEN INTO MIXTURE WITH THE WASTE NITROGENBEFORE SAID WASTE NITROGEN IS COMPRESSED.